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Thyroid Drug Potency is Being Tightened: FDA Changes Specs for Levothyroxine

Synthroid, Levoxyl, Levothroid, Unithroid, and the Generics Are All Affected

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Updated October 04, 2007

Thyroid Drug Potency is Being Tightened: FDA Changes Specs for Levothyroxine
On October 3, 2007, the FDA issued a press release and an online Q&A, announcing tighter new potency specifications for levothyroxine sodium, the thyroid hormone replacement drug that is used to treat hypothyroidism and other thyroid problems. Levothyroxine sodium is the drug's generic name, but it is most often known by the popular brand names Synthroid, Levoxyl, Levothroid or Unithroid.

These new provisions, which go into effect in two years, are designed to ensure a narrower range of potency of levothyroxine throughout its entire shelf life. Specifically, the FDA specifications say that levothyroxine sodium medications will need to maintain potency levels between 95 and 105 percent until their expiration date. This range is tighter than the current standard, which is a range of 90 to 110 percent potency.

The FDA has mandated these changes because they have found in recent surveys that levothyroxine potency is not stable. The FDA evaluated stability data for levothyroxine from all manufacturers, for drugs produced between July 2003 and June 2005. What they found was that the drugs tended to lose potency over time, and some were degrading quite quickly, and losing so much effectiveness that they were close to the 90% minimum potency level by the time they were close to expiration.

Other findings of the FDA:

  • Some levothyroxine packaging -- blister packs, for example -- degrade more quickly than others.
  • Some levothyroxine tablets were stable and lost less than 5 percent of their potency over a 2 year period, while others lost as much as 10 percent of labeled potency in just 9 months.
According to the FDA's press release, Janet Woodcock, MD, of the FDA's Center for Drug Evaluation and Research, has said, "Tightening the potency specifications will ensure that the most vulnerable patients taking thyroid medication will receive the appropriate level of drug therapy needed for their condition."

The FDA says that their new specifications will ensure that levothyroxine products are more stable, have less fluctuation in potency, and will limit the amount they can degrade over the entire shelf life. The ultimate goal is less variability in the products, which in turn, will help ensure more stable thyroid function for patients. This is, according to the FDA, especially important for thyroid cancer survivors, who require stable, suppressive doses of medication to prevent cancer recurrence.

The affected companies include all manufacturers of levothyroxine:

  • Levolet, from Vintage (not marketed in the US)
  • Levo-T, from Alara
  • Levothroid, from Lloyd, distributed by Forest
  • Levothyroxine Sodium, from Genpharm
  • Levothyroxine Sodium, from Mylan
  • Levoxyl, from Jones Pharma/King
  • Synthroid, from Abbott
  • Tirosint, from Institute Biochimique
  • Unithroid, from Jerome Stevens
Interestingly, this issue of the potency and stability of levothyroxine goes back more than a decade. In 1997, the FDA announced that due to long-standing stability and potency problems, levothyroxine products were being classified as "new drugs" and would need to go through the "new drug application" (NDA) process. Until then, levothyroxine, which was introduced in the 1950s, was regulated by the FDA, but had never gone through a formal "approval" process, having been grandfathered in with the natural thyroid drugs in use since the early 1900s. You can read more about the new drug approval process for levothyroxine in an article here at the About.com Thyroid site.

Implications and Recommendations for Patients

If you're a thyroid patient taking levothyroxine, this new requirement won't have any particular effect on you in the short term. You should continue taking your medication as prescribed by your practitioner.

The FDA does advise, however, that to help ensure the best possible potency of your medication, you should store your levothyroxine drugs in a dry place, at room temperature. You should avoid humid or hot conditions -- that means, you should NOT store your levothyroxine in the bathroom. Summertime and hot climates -- as well as the popularity of mail order pharmacies -- raise additional issues. Find out important guidelines on how to protect your thyroid medication from heat damage.

The manufacturers have a two-year deadline to comply with these new specifications. According to the FDA, some of the products already meet the new potency specifications, and there won't be any changes to those products. Other products may have their expiration dates shortened, or may get new packaging to help extend shelf life. Some products may undergo some reformulating in order to improve stability. (I'm looking into whether the specific product data is available.)

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